As shown in FIGS. 3A and 3B, a rotor core 12 of a squirrel-cage rotor used in an induction motor is formed of a laminate of magnetic sheets, such as silicon steel sheets, and comprises a plurality of conductor rods 46 arranged individually in a plurality of slots 44 that are formed in the axial direction along the substantially cylindrical outer peripheral edge, and a pair of end-connector rings 48 which are arranged individually at the axially opposite ends of the core 12 to short-circuit the conductor rods 46 one another.
These conductor rods 46 and the pair of end-connector rings 48 are integrally molded by casting, such as aluminum die casting. A cylindrical axial hole 50 in the center of the rotor core 12 which is integral with molded conductor portion is fixed on a shaft 52 by shrink fitting or the like, whereupon the well-known squirrel-cage rotor is formed.
This conductor molding method is excellent in productivity and facilitates molding of a conductor in a desired shape, so that it has an advantage of being able to improve the characteristics of the motor. Heretofore, it has been widely used for a small-sized induction motors, in particular.
Conductor molding by casting, however, may bring on a problem that shrinkage cavity or gas cavity is caused in a molded conductor. According to the die casing method in which a molten metal, such as aluminum, is loaded at high speed and under high pressure, in particular, the molten metal is liable to form a turbulent flow and trap gas as it is loaded, so that development of blowholes is unavoidable. In the squirrel-cage rotor, moreover, the radial sectional area of each conductor rod is much smaller than the radial sectional area of each end-connector ring. During the casting operation, therefore, the molten metal first solidifies in the slots (in which the conductor rods are arranged to form the squirrel-cage rotor) that axially penetrate the laminate of magnetic sheets, such as silicon steel sheets, so that a sufficient pressure cannot be transmitted to those end-connector ring portions which solidify after a delay (especially those end-connector ring portions which are located on the side remoter from the pouring gate), and blowholes tend to be easily formed. Since a conductor having blowholes is low in mechanical strength, it is difficult to apply a squirrel-cage rotor including such a conductor to a high-torque induction motor or high-speed induction motor with a speed of tens of thousands of rpm.
Already proposed is a casting method in which a molten metal filling an end-connector ring cavity between a mold and a rotor core is locally pressurized independently of the filling pressure, in order to prevent the formation of blowholes in the conductor (end-connector ring in particular). According to this method, the molten metal is loaded from one end-connector ring cavity, which communicates with a pouring gate, into the other end-connector ring cavity, on the side remoter from the pouring gate, through a plurality of slots, and first solidifies in the slots with a small sectional area, thereby cutting off applied pressure from the end-connector ring cavity on the side remoter from the pouring gate. As this is done, pressure is additionally applied to the molten metal in the end-connector ring cavity on the side remoter from the pouring gate before this molten metal solidifies. Thus, the end-connector ring on the side remoter from the pouring gate can be molded under a desired pressure, so that the formation of blowholes can be prevented.
This local-pressurization casting method requires use of local pressurization means that can apply a uniform pressure to the whole end-connector ring on the side remoter from the pouring gate. For ease of manufacture, however, many molds for squirrel-cage rotors are generally constructed so as to be able to open and close in the same direction as the axial direction of the rotor core. In installing the local pressurization means on the side remoter from the pouring gate of one such mold, the installation space for the local pressurization means must be provided in the mold so as not to influence external structures, such as a mold supporting structure. Thus, the mold is increased in thickness and external dimensions, so that its manufacture is difficult, and its practicability is ruined.
Further, the molds for squirrel-cage rotors must be provided with a retaining structure for firmly holding the rotor core in a predetermined position in the mold, resisting the molten metal filling pressure, on the side remoter from the pouring gate. How to arrange this retaining structure, together with the aforesaid local pressuring means, orderly on the side remoter from the pouring gate of the mold is also a problem to be solved.
Also proposed is a local pressurization method in which the gate-side portion of the mold is designed so as to be movable in the axial direction, and the movable pouring gate-side portion of the mold is axially moved together with the rotor core by the filling pressure after the molten metal is loaded from the pouring gate-side end-connector ring cavity into the end-connector ring cavity on the side remoter from the pouring gate through the slots, whereby pressure is additionally applied to the end-connector ring on the side remoter from the pouring gate. Although the local pressurization means need not be located on the side remoter from the pouring gate, according to this method, the construction of the mold is complicated, and moreover, a special casting machine (injection molding machine) with a very high pressure is needed to move a movable part of the mold together with the rotor core, so that the installation cost is very high.